If in an e.g. wireless communication system modulation methods are used, which result in a non-constant envelope of the high-frequency carrier signal, all signal processing components after the modulator must have a sufficient linearity. This demand is difficult to fulfill, especially in power amplifier stages which should operate with high efficiency.
When pulse amplitude modulation methods are used, the spectral efficiency will deteriorate due to the non-linearity of the amplifiers. The reason for this is to be seen in the non-linear amplitude output characteristics of an amplifier, which leads to an AM/AM conversion; the phase drift of an output signal of an amplifier relative to the phase of an input signal produces, in addition, intermodulation components, which is also referred to as AM/PM conversion. The AM/AM and the AM/PM conversion must, however, be prevented by suitable linearization methods. If this is not done, the spectral efficiency of the modulation method used as well as the signal-to-noise ratio will deteriorate. In digital transmission systems this can cause a substantial increase in the bit error rate during the transmission.
Especially in transmission systems making use of a QPSK modulation (QPSK=Quatenary Phase Shift Keying), as is e.g. the case with mobile telephone systems, it is particularly important that the non-linear region of power amplifiers is utilized. In view of the fact that, e.g. in mobile telephones, the available power supply is limited, i.e. a larger storage battery makes the mobile telephone much heavier and much more expensive, the final stage amplifier must operate with the highest possible efficiency, but this is not possible in its linear amplification region. High-efficiency power amplifiers are therefore preferably operated in their non-linear region near saturation, and this causes non-linear distortions entailing the problems described.
Modulation methods requiring a linearization in addition to the PAM modulation, are, fundamentally, all multi-carrier methods (e.g. COFDM for digital broadcasting) and all pulse amplitude modulation methods which comprise, in addition to the QPSK modulation, also the QAM modulation. Further possibilities of use for the predistortion of a non-linear channel exist in the field of base stations of mobile telephone systems when several frequency channels are supplied to a final transmitter stage in parallel.
Hence, there is a need for a suitable linearization method based on predistortion. This method could be used whenever a non-linear channel is to be linearized in general. The linearization of a non-linear channel should additionally permit the frequency region of the input signal into the non-linear channel that is to be equalized to differ from the frequency region of the output signal from the non-linear channel. The linearization must therefore not be limited to mere amplifier stages, but it must also be possible to carry out frequency conversions within the non-linear channel.